Methods of mechanically plating metal objects with copper and alloys thereof



United States Patent 3,251,711 METHODS OF MECHANICALLY PLATING METAL ggJECTS WITH COPPER AND ALLOYS THERE- Rolfe Pottberg and Erith T. Clayton, Baltimore, Md..,

assignors to Peen Plate, Inc., Baltimore, Md., a corporation of Maryland No' Drawing. Filed June 20, 1962, Ser. No. 203,705 6 Claims. (Cl. 117-109) This invention relates to the general system of surfacing one metal with another through the use of mechanically .appliedattritive forces and the employment of the surfacing metal in finely comminuted condition usually in carefully specified chemical or physical environments. Some form of impacting medium is nearly always used, in the form of a plurality of metallic or non-metallic granules.

v It is a general object of this invention to describe a means for more controllably and predictably applying films of relatively noble metals such as copper or alloys thereof withsuch metals as zinc, tin and nickel, to the surfaces of metallic objects.

. Specifically it relates to the nature of the impacting medium granules used to accomplish the .plating of objects by this general method of plating (herein referred to as mechanical plating), and describes improved, novel types of impacting granules useful in the plating of such metals onto the surfaces of less noble metals; noting the advantages and improvements which are attained and previously encountered problems that are overcome in commercial operation.

It is another specific object of this invention to show 7 how less expensive forms of the coating metal may be substituted in part for the relatively expensive, finely powdered form of the metal to be applied to the surfaces.

By impact medium is meant granules of hard, abrasively resistant material which may be either metallic or non-metallic and which are smaller than the objects being plated and much larger than the particles of coating metal powder; and which serve to augment, increase, intensify and distribute the pressures applying the particles of coating metal powder to the surfaces of the metal articles being plated;

this instance are metals having a lower solution potential than the metals of the metallic surfaces to which they are being applied; such as, generally, metals lower in the electromotiveseries than iron and steel, and most particularly copper and commercial alloys thereof such as brass, bronze, Monel, etc.

The non-metallic impacting granules are most frequently employed in mechanical plating such as various types of vitreous, ceramic or mineral granules; but if the metallic objects being coated are a large mass of heavy objects, the non-metallic granules may be gradually broken or ground up and as a result thereof become unsuitable asimpacting media and will not be useful in laying down a good surface on the metal objects. i

Additionally, the surfaces of non-metallic granules are not themselves susceptible to metal coating with particles of the metal being applied, and consequently apply the coating metal particles directly to the objects as discrete particles. Such finely divided particles of copper and nickel and their alloys are extremely stress hardenable and except under conditions of severest mechanical applications, multi-layers of these particles are difli-cult to compact and to integrate into a smooth metallurgically continuous film through the use of the norm-ally used nonmetallic impacting bodies or media.

In consequence of these two facts coatings of these metals laid down by non-metallic media while tending to be thicker tend also to be rougher, less polished, and. to

I have more pores perunit of thickness.

The coating of more noble metals such as brass, is usually done to impart a desired surface appearance to ferrous objects, and because of porosity coupled with a cathodic relationship to the substrate often rely upon some further means such as lacquers, chroma-te films and the Iiketo confer added resistance to corrosion under prac tical conditions.

Because of the foregoing, metallic impacting granules have most frequently been employed for brass plating in this particular manner. Naturally, the most commonly, commercially available type of metallic such granule consisting, usually, of iron or steel shot, cracked steel shot, cut steel wire, was employed. These granules are susceptible to plating with the metal to be applied. It is believed that in this case some of the coating may be transferred as a result of the preliminary acquisition of a coating by the impacting granules (which do not become effective until they themselves have been coated with the metal to be plated) to the objects being plated; but in. any case the finely divided coating metal particles as of brass are more susceptible to interception and dis-tortion by colliding, metallic impacting granules, being attracted to the surfaces thereof. The opposite is the case with an impacting granule having a, non-metallic surface.

In consequence of this the coating metal particles become distorted and flattened and assume a brilliant flakelike form. When such flakes subsequently do become implanted in parallel rows on the surfaces of the objects the resulting coating is, of course, smoother, more polished and less porous in character. It-has been determined, however, that such metallic impacting granules have a significant'commercial disadvantage. This arises from the fact that, as noted, they will not apply a coating to the objects to be plated until they .themselves are adherently plated with the coating metal but, unfortunately, the plating on the impacting granules is not stable under practical storage conditions in aqueous environments.

It has been determined that it was extremely difficult if not practically impossible to obtain completely uniform commercial results from such operations because of factors arising during storage of the coated metallic impacting medium in aqueous solution, permitting the solution to contact and deteriorate the coating because the said coating is suificiently porous to allow the solution to contact not only the coating but the base metal of the impacting media and because of the electrolytic relationship between coating and substrate this caused a destruction of the bond between the coating films on the impacting media, as well as an accumulation of ferrous salts in the solution. It should be noted in passing that it is usually uneconomical to dry the coating impacting media between operations.

As a consequence of the foregoing, the time elapsing between operations becomes a significant process variable, to the extent that storage over a weekend, for example, often requires a complete resurfacing operation to be performed on the impacting granules before.-

uniform, consecutive results can again be obtained, even if thoroughly rinsed and stored in clean Water. Because of a lesser tendency of more noble plating metal particles to deposit on less noble surfaces in this generic method, it is usually desirable to preliminarily surface such ferrous impacting granules with more noble metal films (such as those which can be applied by chemical immersion treatments in salts of such metals). Thereafter such impacting granules more readily pick up a film of the metal to be coated from the aqueous slurry with particles such as of brass suspended therein, and transfer the metal thereafter to the surfaces it is desired .to film..

Another factor militating against consistent performance when iron or steel impact granules are utilized, and

which is also a function of the period of inactivity be-. tween operations, is the accumulation of ferrous salts in the aqueous solutions employed if such solutions are to be reused after replenishment with further chemicals and metal powders, instead of simply being discarded and replaced with fresh solutions and metal powders.

It has'been determinedthat impacting media consisting 'Metals such as brass, bronze, copper etc., are not normally readily available commercially 'in a physical condition lending them to use as impacting granules except at high cost; but certain by-product forms were found to be available at scrap metal prices. Illustratively these may. consist of brass and copper clippings, punching's, etc., from various commercial fabricating operations.

An important additional advantage is found in the impacting medium made from copper or its alloys when used in the coating operation in that the medium may provide significant, additional quantities of metal to the coating on the metallicrobjects, thereby reducing the, amount of the finely powdered copper and its alloys or other metals heretofore used in the coatingoperation. This naturally occurs in proportion to the surface area to weight ratio of the granules. Thus, a material saving in the cost of coating the metallic objects can be accomplished, because a smaller. amount of higher-priced powdered copper or its alloys can 'be used, and much of the metal of the coating derived from scrap pieces of copper and its alloys used as impacting granules. Also,

brass in the form of cheaper gamma brass, not suited I for other purposes, may be used.

A further saving is obtained by permitting reuse of the solution, simply replenishing the impacting medium of brass, etc., in subsequent coating operations, plus a less amount of chemicals to promote the coating, and also diminishing the amount of the powdered copper and its alloys below the point otherwise used in such coating operations. The reused solution remains purer and free of any salts of the metal of the impacting media, and can be reused more times than if ferrous granules are employed.

In all of-the examples set forth hereinafter, small. scale equipment was used and in which certain factors were constant throughout the examples. Even though small scale equipment was used, such results as were obtained can be projected to produce comparable results in a commercial scale operation, except that the proportion of coating metal derived from the impacting medium is less on a smaller scale of operation.

The common factors in all of the following examples were as follows:

A hexagonal tumbler of 4700 cc.,capacity having a non-metallic interior lining was used. This tumbler was rotated at a peripheral speed of 100 ft. per minute for two hours and was charged with 4 lbs. of clean ferrous objects and 1200 ccs. of metallic impacting granules varying in major dimensions from to A"; and in minor dimensions from to 'f. metal powder added consisted of stated quantities of commercially available atomized bras powder of 30%' zinc/ copper of minus 395 US. standard mesh. Also, 1200 ccs. total liquids were employed, consisting of water and hereinafter stated quantities of the following solution, selected as typical of many solutions employable;

Grams Anhydrous citric acid 1000 Polyoxyet-hylene oleic amide 400 Sufiicient water to make 4000 ccs.

Examples (2) However, when the impacting granules of crushed steel shot were surfaced with copper by immersion in an acidified solution of copper sulfate and then placed in the mill as indicated above, both the impacting granules and the ferrous objects become uniformly filmed with a 7 continuous and adherent film of brass up to .0003 in thickness.

(3) Steel impacting granules were first mechanically or chemically etched as by abrasion or acid treatment to increase their mechanical retention of implanted brass faced with brass. Thereafter, the mill was prepared and operated as in Example 1, using the coated impacting granules, and the result was that the ferrous objects were plated with'brass in a continuous and adherent film up to .0003" in thickness.

(4) 'In this example, the impacting granules of No. 2, that is those coated by immersion, were stored in tap water for twenty-four hours prior to use. Although little perceptible change in the surface appearance of the granules occurred, the film of brass plated on the ferrous objects was perceptibly thinner, having some of the appearance of the underlying steel substrate.

(5) Here the impacting granules of Example 3 were likewise stored for twenty-four hours prior to use and the. result obtained was the same as in Example 4.

(6 and 7) However, when the impacting medium of Examples 4 and Swere again immediately reused in an identical manner for a second time, the result in each instance is the same result as obtained in Examples 2 and 3.

(8 and 9) Here the impacting medium as treated in Examples 4 and 5 was stored for one week in tap water. At the end of one week, the tap water was visibly and heavily discolored with ferrous salts and the brass :Exceptwhere noted the r ing the same, the brass was plated ,on the ferrous objects in a continuous and adherent film less than .0003" in thickness.

(11 and 12) The brass impacting granules of Example were stored as in Examples 4 and 8, that is, for

twenty-four hours and for one week, and when used in a plating operation, the plating of brass on the ferrous objects was as specified in Example 2, namely up to .0003".

(13 and 14) The impacting granules of Examples 2 and 3, that is the steel shot surfaced with copper by immersion or by attrition were used in a plating bath in which the solution was reduced to 15 ccs. and the brass powder reduced to 15 grams. The result obtained was a very thin film on the ferrous objects, whichfihn was so thin as to be commercially unacceptable.

(15) A mill in which the brass impacting granules of Example 10 were used in a reduced solutionof 15 ccs. and the brass powder reduced to 15 grams nevertheless obtained a result of a commercially satisfactory coating of the thickness of Example 2.

(16) In this test, the brass impacting medium was crushed steel shot of. Example 1, with all other factors be- 4 used with the plating solution reduced to 15 ccs. but no brass powder was added. The result of this example was a very thin coating of brass with some of the underlying steel substrate being visible.

In both Examples 15 and 16, the initial weight of the brass impacting media was reduced.

(17) The remaining solutions of Examples 2 and 3 were reused once after addition of another 15 grams of brass powder and 15 ccs. of solution, with the volume restored to 1200 ccs. with tap water; and other conditions bleing the same, the resultant plating was transparently t in.

(18) When the solution of Example 17 was used again for a second time with similar additions, there resulted no plating on the new, ferrous objects.

(19) When brass impacting media is substituted for the cracked coated steel shot for the impacting media of Examples 17 and 18 and the solutions used as in Examples 17 and 18, the resultant plating on the ferrous objects was as that set forth in Example 2.

It has also been determined that impacting granules of copper, in this case consisting of tetragonally shaped copper clippings known as nail whiskers and resulting from the operation forming the points in the manufacture of copper nails, behaved, initially, in-the same manner as the copper surfaced, steel granules, of Example 2. However, the results obtained therewith following periods of storage for twenty-four hours and for one week were similar to those obtained with the brass medium rather than with the medium of Example 2; and the results obtained when the solutions were reused, merely being replenished with smaller quantities of metal powders and chemicals, were comparable to those obtained with the all-brass medium.

(20) In this particular example, lead impacting granules were substituted for the cracked steel shot of Example 1, with the result that there was a perceptable but very light plating on the ferrous objects and that the lead impacting media became surfaced with the brass powder.

(21) In this example, the lead impacting medium was treated as were the steel shot media of Examples 2 and 3 and immediately used, in which the resultant plating was the same as in Examples 2 and 3.

(22) In this example, the lead impacting medium treated in accordance with Examples 2 and 3 and stored for twenty-four hours in tap water and then used, resulted in a plating of brass on the ferrous objects as in Examples 2 and-3. Here, however, there was an accumulation of insoluble lead salts in the stored solution which apparently, from the results obtained, did not affect the efficiency of the impacting medium.

(23) Here, an impacting medium consisting of an alloy of 50% zinc/50% copper was substituted for the impacting medium of 30% zinc/70%v copper, and the results obtained were comparable to the results obtained for the 30/70% alloy.

(24) In the plating of nickel on ferrous objects by using impacting granules of nickel, or of lead which has been surfaced with nickel by placing the lead medium in a mill with nickel particles and first coating the lead with the nickel, and in which a nickel powder of minus 395 mesh is used, the result obtained was a very thin, approximately .0001 but visually continuous and adherent film of bright nickel on the ferrous objects.

(25) Three lbs. of small zinc base die casting parts was added to the mill wherein the impacting granules consisted of bare steel granules 40 ccs. of a modified promoter solution consisting of 29% by weight of citric acid and 71% by weight of inhibitor was added, together with the brass powder and water. After two hours of rotation there was no coating on the zinc base objects.

(26) The above described experiment was repeated exactly except that steel granules very well plated with brass were substituted for the bare steel granules and after rotation for two hours the zinc base die castings were found to be coated with a smooth, continuous brass coating of approximately .0003".

Having described the invention, there is now set forth the claims:

1. In a method of converting the surface of metal objects with a film of a plating metal wherein the film is formed as the result of mechanical attritive forces imparted by an aqueous fluid slurry, the improvement which comprises utilizing an aqueous fluid slurry which contains a suspension of finely comminuted particles of the plating metal and a plurality of metal impacting granules smaller than a metal object to be plated and larger than the plating metal, said plating metal having a solution potential lower than that of the metal object to be plated, said metal impacting granules having a solution potential no greater than that of the plating metal, and said metal object being at least substantially stable in its aqueous environment.

2. A method according to claim 1 wherein the plating particles and impacting granules are each a metal selected from the group consisting of copper, copper alloys and nickel and wherein the solution potential of the impacting granules metal is no higher than that of the plating particle metal.

3. A process for covering the surfaces of metal objects as set forth in claim 1 in which the objects are ferrous and the particles of plating metal are selected from the group consisting of copper, copper alloys, and nickel, and the granules of the impacting medium are selected from the metal of the same group.

4. A process for covering the surfaces of metal objects as set forth in claim 1 in which the plating metal particles and the impacting granules are of nickel.

5. In a process for plating the surfaces of metallic objects with films of brass by agitation in aqueous slurries containing suspensions of metallic objects at least substan- 1 tially stable in the aqueous environment and particles of brass, the improvement comprising the addition to the slurry of impacting media consisting of granules of gamma brass larger than the particles of brass and smaller than the objects.

6. A process of mechanically plating the surfaces of metal objects with a film of another-metal having a lower solution potential than said object comprising exposing at least one metal object to mechanical attritive forces imparted by a fluid aqueous slurry containing: (a) an organic acid, (b) very small, finely comminuted particles of a plating metal having a solution potential lower than the solution potential of the metal object; and (c) a plurality of metal impacting granules smaller than the metal object and much larger than the plating metal particles UNITED STATES PATENTS 5/ 1953 Clayton 117109 9/1954 Clayton l17109 9/1961 Clay l17--109 8 FOREIGN PATENTS 534,888 11/ 1941 Great Briatin. 740,075 11/1955 Great Britain. 740,104 11/ 1955 Great Britain.

OTHER REFERENCES Merriman: A Dictionary of Metallurgy, Macdonald and Evans Ltd., London, 1958, p. 106.

10 RICHARD D. NEVIUS, Primary Examiner.

R. S. KENDALL, J. S. SPENCER, Examiners. 

1. IN A METHOD OF CONVERTING THE SURFACE OF METAL OBJECTS WITH A FILM OF A PLATING METAL WHEREIN THE FILM IS FORMED AS THE RESULT OF MECHANICAL ATTRITIVE FORCES IMPARTED BY AN AQUEOUS FLUID SLURRY, THE IMPROVEMENT WHICH COMPRISES UTILIZING AN AQUEOUS FLUID SLURRY WHICH CONTAINS A SUSPENSION OF FINELY COMMINUTED PARTICLED OF THE PLATING METAL AND A PLURALITY OF METAL IMPACTING GRANULES SMALLER THAN A METAL OBJECT TO BE PLATED AND LARGER THAN THE PLATING METAL, SAID PLATING METAL HAVING A SOLUTION POTENTIAL LOWER THAN THAT OF THE METAL OBJECT TO BE PLATED, SAID METAL IMPACTING GRANULES HAVING A SOLUTION POTENTIAL NO GREATER THAN THAT OF THE PLATING METAL, AND SAID METAL OBJECT BEING AT LEAST SUBSTANTIALLY STABLE IN ITS AQUEOUS ENVIRONMENT. 